1. Field of the Invention
The present invention relates to an optical disc drive for use to read out information from an information storage medium such as an optical disc by an optical technique (i.e., by using a light beam such as a laser beam). More particularly, the present invention relates to a method of controlling the convergent lens of an optical head included in the optical disc drive.
2. Description of the Related Art
An optical disc drive performs a tracking control by getting its convergent lens moved by a tracking actuator in the disc radial direction (i.e., across the tracks on the disc). The tracking actuator includes a fixed portion and a movable portion, which is attached to the convergent lens. The fixed portion and the movable portion are connected together with four wires or elastic members of rubber, for example. Each of the fixed and movable portions includes a coil and a permanent magnet. When a predetermined amount of current is allowed to flow through the coil, an electromagnetic field is generated between the coil and the permanent magnet, thereby moving the convergent lens across the tracks on the disc (or vertically to the tracks). In such a tracking control operation, the position of the convergent lens is controlled in such a manner as to prevent the laser beam, emitted from the optical pickup, from going off the tracks.
In carrying out a seek operation, the optical disc drive stops the tracking control operation and moves the optical head in its entirety (i.e., including the tracking actuator) toward a target track on the disc in the disc radial direction. The distance that the optical head has gone is calculated based on the number of tracks that the focal points of the light beam have crossed on the disc.
In such a seek operation, if the optical head is rapidly moved in the disc radial direction, the convergent lens thereof is subject to inertial force and vibrates. To reduce such a vibration, the conventional optical disc drive disclosed in Japanese Patent No. 2734884 includes a position sensor for detecting the shift of the convergent lens from its neutral position in the disc radial direction. This conventional apparatus obtains a velocity signal by differentiating the output signal of the position sensor and supplies the velocity signal to the tracking actuator, thereby canceling the inertial force.
An alternative method of reducing the vibration of the convergent lens without using such a position sensor is proposed in Japanese Laid-Open Publication No. 9-274726, for example. This method will be described in detail with reference to FIGS. 12 and 13. FIG. 12 is a block diagram showing a detector for detecting a light beam that has been emitted from the optical head and then reflected from the disc. Portion (a) of FIG. 13 is a graph showing how the position of the convergent lens changes with respect to its reference position while an optical disc drive, including the detector shown in FIG. 12, performs a seek operation. Portions (b), (c) and (d) of FIG. 13 show the waveforms of respective signals obtained at respective components of the detector.
As shown in FIG. 12, the photodetector 1001 is divided into four regions A, B, C and D to detect the light beam that has been reflected from the disc. The regions A and D are located closer to the outer edge of the disc, while the regions B and C are located closer to the inner edge of the disc. In this case, the output signals of the two outer regions A and D are added together at an adder 1002 to obtain a sum signal A+D. In the same way, the output signals of the two inner regions B and C are also added together at an adder 1003 to obtain a sum signal B+C. While the convergent lens is changing its positions as shown in portion (a) of FIG. 13, these two sum signals A+D and B+C change their levels as shown in portions (b) and (c) of FIG. 13, respectively.
The output signals of the adders 1002 and 1003 are supplied to peak holders 1004 and 1005, respectively. The peak holders 1004 and 1005 hold the peak values of the two sum signals A+D and B+C, thereby outputting signals PH(A+D) and PH(B+C), respectively, as shown in portions (b) and (c) of FIG. 13. Next, these signals PH(A+D) and PH(B+C) are supplied to a subtractor 1006, which obtains the difference between these two signals and outputs the difference as a lens position detection signal LE as shown in portion (d) of FIG. 13. By operating the tracking actuator such that the lens position detection signal LE equals zero during this seek operation, the inertial force can be canceled and the vibration of the convergent lens can be reduced.
However, the conventional optical disc drive described above needs either the position sensor or peak holders, thus increasing the manufacturing cost of the drive, the size of the optical head or the number of pins of the LSI unintentionally. Also, if an optical disc with periodically wobbled track grooves such as a CD-R or a CD-RW is read by an optical disc drive including the detector shown in FIG. 12, then the position of the convergent lens may not be detectable accurately. This is because when the optical head is moved slowly over such an optical disc, the lens position detection signal might be affected by the presence of those wobbled track grooves.